Add guidelines on prioritizing high priority tickets

Why is this change being made?

This change is being made to add clear guidelines around prioritization when working on tickets. The main adjustment is to prioritize High priority tickets over Normal and Low priority tickets, regardless of which ticket is next-to-breach. This applies to paid user tickets only.

More context and details are in the following:

• Workflow change issue: gitlab-com/support/support-team-meta#5272 (closed)
• Idea trial issue: gitlab-com/support/support-team-meta#5178 (closed)

Author Checklist

• Provided a concise title for this Merge Request (MR)
• Added a description to this MR explaining the reasons for the proposed change, per say why, not just what
  • Copy/paste the Slack conversation to document it for later, or upload screenshots. Verify that no confidential data is added, and the content is SAFE
• Assign reviewers for this MR to the correct Directly Responsible Individual/s (DRI)
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to one of the people listed in the Maintained by section on the page being edited
  • If your manager does not have merge rights, please ask someone to merge it AFTER it has been approved by your manager in #mr-buddies
  • The when to get approval handbook section explains the workflow in more detail
• If the changes affect team members, or warrant an announcement in another way, please consider posting an update in #whats-happening-at-gitlab linking to this MR
  • If this is a change that directly impacts the majority of global team members, it should be a candidate for #company-fyi. Please work with internal communications and check the handbook for examples.

---